Organic electroluminescent display and driving method thereof

ABSTRACT

A pixel circuit of an organic EL display includes an EL device, first and second switching devices, a driving thin film transistor, and a capacitor. 
     The first switching device switches data voltages applied to data lines in response to the selection signal applied to a scan line and the second switching device connects gate and drain of the driving thin film transistor in response to a compensation signal applied to a compensation line. The driving thin film transistor supplies electric current to the organic EL device in response to the data voltage inputted to a gate from the first switching device and the capacitor maintains the data voltage applied to the gate of the driving thin film transistor for a predetermined period. At this time, the characteristic deviation of the transistor is compensated by connecting the gate and the drain of the driving thin film transistor by applying the compensation signal to the compensation line before applying the data voltage, and then the data voltages are applied to the data lines after cutting off the compensation signal. In this manner, the characteristic deviation of the driving thin film transistors can be compensated.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to an electroluminescent (referred to as“EL” hereinafter) display and driving method thereof.

(b) Description of the Related Art

An organic EL display is a display emitting light by electricallyexciting a fluorescent organic material, and it displays images byvoltage-driving or current-driving M×N organic luminescent cells. Anorganic luminous cell includes an anode (ITO), an organic thin film, anda cathode layer Metal. The organic thin film is formed ofmultiple-layers including an emitting layer (EML), an electron transportlayer (ETL), and a hole transport layer (HTL) for improvinglight-emitting efficiency by balancing electrons and holes, and alsoincludes separate an electron injecting layer (EIL) and a hole injectinglayer) (HIL).

The organic luminescent cells are driven by a simple matrix (or passivematrix) type and an active matrix type using thin film transistors(TFTs). The simple matrix driving is to select cathode lines and anodelines crossing each other, while the active matrix driving to connectTFTs and capacitors to ITO pixel electrodes and to store voltages intothe capacitors.

FIG. 11 is a circuit diagram of a conventional pixel circuit of arepresentative one of N×M pixels, for driving an organic EL device usingTFTs. Referring to FIG. 11, the organic EL device OELD is connected to adriving transistor Mb for supplying light-emitting current. The amountof current driven by the driving transistor Mb is controlled by datavoltage supplied through a switching transistor Ma. A capacitor C forkeeping the supplied voltage for a predetermined time is connectedbetween a source and a gate of the transistor Mb. A gate of thetransistor Ma is connected to the n-th scan line, and the source thereofis connected to a data line.

Seeing an operation of a pixel with the structure, a selection signalapplied to the gate of the transistor Ma turns on the transistor Ma, andthen the data voltage V_(DATA) is applied to the gate A of the currentdriving transistor Mb through the data line. Then, the current flowsinto the organic EL device OELD through the transistor Mb in response tothe data voltage V_(DATA) applied to the gate of the transistor Mb, andthe organic EL device OELD emits light.

The amount of the current flowing in the organic EL device is given byEquation 1.

$\begin{matrix}{I_{OLED} = {{\frac{\beta}{2}\left( {V_{GS} - V_{TH}} \right)^{2}} = {\frac{\beta}{2}\left( {V_{DD} - V_{DATA} - V_{TH}} \right)^{2}}}} & (1)\end{matrix}$where I_(OLED) is a current flowing in the organic EL device, V_(GS) isa gate-source voltage of the transistor Mb, V_(TH) is a thresholdvoltage of the transistor Mb, V_(DATA) is a data voltage, and β is aconstant.

According to Equation 1, the current supplied to the organic EL devicedepends on the applied data voltage V_(DATA) in the pixel circuit shownin FIG. 11, and the organic EL device turns to be luminescent inresponse to the supplied current. Here, the applied data voltageV_(DATA) has multiple values in a predetermined range.

However, the conventional pixel circuit has a drawback in that it causesthe non-uniform brightness of the panel because of the characteristicdeviation of the thin film transistors caused by the unevenness ofmanufacturing process.

To compensate for this problem, it is proposed to use additional thinfilm transistors in a pixel circuit. In this pixel circuit, however, aaperture ratio of the panel decreases due to the increase of the numberof the thin film transistors and it takes so long time to charge thecapacitor for low gray scale.

SUMMARY OF THE INVENTION

A motivation of the present invention is to provide a pixel circuitcapable of compensating characteristic deviation of driving thin filmtransistors. Another motivation of the present invention is to reducetime required for charging a capacitor.

To achieve the above motivation, a pixel circuit of the presentinvention includes an additional compensation transistor.

According to one aspect of the present invention, an organic EL displayincludes a plurality of data lines, a plurality of scan lines, and aplurality of pixel circuits, each pixel circuit provided at a pixel areadefined by two adjacent data lines and two adjacent scan lines. The datalines transmit data voltages representing image signals, the scan linestransmit selection signals, and the compensation lines transmitcompensation signals.

Each pixel circuit includes an organic EL device, first and secondswitching devices, a first thin film transistor, and a capacitor. Theorganic device emits light according to the amount of the currentapplied thereto. The first switching device switches the data voltageapplied to the data line in response to the selection signal applied tothe scan line and the second switching device connects gate and drain ofthe first thin film transistor in response to the compensation signalapplied to the compensation line. The first thin film transistorsupplies electric current to the organic EL device in response to thedata voltage inputted to its gate through the first switching device andthe capacitor maintains the data voltage applied to the gate of thefirst thin film transistor for a predetermined period.

It is preferable that the compensation signal is applied before the datavoltage is applied to the data line, and the data voltage is applied tothe data line after the compensation signal applied to the compensationline is cut off.

Different supply voltages are applied to sources of the first thin filmtransistors of red, green, and blue pixels.

The pixel circuit may further include a second capacitor for uniformlymaintaining the voltage applied to the gate of the first thin filmtransistor during the application of the data voltage and the secondcapacitor is preferably connected to the first capacitor in series.

Preferably, the first switching device is a second thin film transistorhaving three terminals, the three terminals of the second thin filmtransistor including a gate connected to the scan line and other twoterminals connected to the data line and the capacitor, respectively,and the second switching device is a third thin film transistor havingthree terminals, the three terminals of the third thin film transistorincluding a gate connected to the compensation line and other twoterminals connected to the gate and the drain of the first thin filmtransistor.

The first thin film transistor may be a first conduction type transistorand the second and the third thin film transistor may be secondconduction type transistors. Alternatively, the first thin filmtransistor is a first conduction type transistor and the second and thethird thin film transistors are different type transistors.Alternatively, the first to the third thin film transistors are the sametype transistors.

According to another aspect of the present invention, a method fordriving an organic EL display is provided. The method applies aselection signal for selecting some of a plurality of pixel circuits isapplied to a scan line. A compensation signal for switching thin filmtransistors to connect gates and drains is applied to the pixelcircuits. Next, data voltages representing image signals are applied todata lines after cutting off the compensation signal, and electriccurrents are supplied to organic EL devices by transmitting the applieddata voltages to the gates of the thin film transistors.

At this time, the selection signal may be applied prior to thecompensation signal or at the same time with the compensation signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view of an organic EL display according to anembodiment of the present invention.

FIG. 2 is a schematic circuit diagram of a pixel circuit according to afirst embodiment of the present invention.

FIG. 3 is a driving-timing diagram of a pixel circuit according to thefirst embodiment of the present invention.

FIG. 4A is a graph illustrating current-voltage characteristic curves ofa driving transistor and an organic EL device in a circuit according tothe first embodiment of the present invention, and FIG. 4B is a graphillustrating current-voltage characteristic curves of a typicaltransistor and a typical organic EL device.

FIGS. 5, 7 and 9 are schematic circuit diagrams of pixel circuitsaccording to second, third, and fourth embodiments of the presentinvention.

FIGS. 6, 8 and 10 are driving-timing diagrams of the pixel circuitsaccording to the second, the third, and the fourth embodiments of thepresent invention.

FIG. 11 is a schematic circuit diagram of a pixel circuit for aconventional organic EL display.

DERAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. The present invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein.

Now, organic EL displays and driving methods thereof according toembodiments of the present invention will be described with reference tothe accompanying drawings.

First, an organic EL display according to an embodiment of the presentinvention will be described with reference to FIG. 1.

FIG. 1 is a schematic plan view of an organic EL display according to anembodiment of the present invention.

As shown in FIG. 1, the organic EL display includes an organic ELdisplay panel 100, a scan driver 200, and a data driver 300.

The organic EL display 100 includes a plurality of data lines 110 fortransmitting data voltage representing image signals, a plurality ofscan lines 120 for transmitting selection signals, a plurality ofcompensation lines 130 for transmitting compensation signals, and aplurality of pixel circuits 140. Each pixel circuit 140 is provided on apixel area defined by two adjacent data lines 110 and two adjacent scanlines 120. Also, different voltages VDD_(R), VDD_(G), and VDD_(B) areapplied to respective red (R), green (G), and blue (B) pixel circuits140.

The scan driver 200 includes a scan driving units 220 for applying theselection signals to the scan lines 120 and another scan driving unit230 for applying the compensation signals to the compensation lines 130.The data driver 300 applies the data voltages V_(DATA) to the data lines110.

Pixel circuits of an organic EL displace according to embodiments of thepresent invention will be described in detail with reference to FIGS. 2to 10 hereinafter.

FIG. 2 is a schematic circuit diagram of a pixel circuit according to afirst embodiment of the present invention, and FIG. 3 is adriving-timing diagram for the pixel circuit according to the firstembodiment of the present invention. FIG. 4A is a graph illustratingcurrent-voltage characteristic curves of a driving transistor and anorganic EL device according to the first embodiment of the presentinvention, and FIG. 4B is a graph illustrating current-voltagecharacteristic curves of a typical transistor and a typical organic ELdevice.

As shown in FIG. 2, a pixel circuit 140 according to the firstembodiment of the present invention includes an organic EL device OELD,a switching transistor M1, a compensation transistor M2, a drivingtransistor M3, and capacitors C1 and C2.

The organic EL device OELD emits light corresponding to the currentapplied thereto. The transistor M3 includes a source connected to asupply voltage VDD, a drain connected to the organic EL device OELD, anda gate supplied with data voltages from a data line, and apply thecurrents corresponding to the data voltages to the organic EL deviceOELD.

The transistor M1 includes a gate, a drain, and a source connected to ascan line 120, a data line 110, and a node P1 between the capacitors C1and C2 and transmits the data voltages V_(DATA) to the transistor M3 inresponse to a selection signal SEL1 applied to the scan line 120. Thetransistor M2 includes a drain and a gate connected to the gate and thedrain of the transistor M3, respectively, and a gate connected to acompensation line 130 and compensates the characteristic of thetransistor M3 in response to a compensation signal SEL2.

The capacitors C2 and C1 are connected in series between the supplyvoltage VDD and the gate of the transistor M2 and maintains the datavoltage applied to the gate of the transistor M3 for a predeterminedperiod. The capacitor C2 is connected between the supply voltage VDD andthe drain of the transistor M1.

The operation of the pixel circuit according to the first embodiment ofthe present invention will be described with reference to FIGS. 3 and 4.

Referring to FIG. 3, in an initialization step S1, when the transistorM1 turns on by the selection signal SEL1 in a high level, the voltage atthe node P1 is set to have an initial voltage level V_(DATA) _(—) _(IN1)of the data voltage.

In a following compensation step S2, if the transistor M2 turns on bythe compensation signal SEL2 in a high state during the turn-on of thetransistor M1, the gate and the drain of the transistor M3 are connectedto each other (to be in a diode connection) to perform a function of adiode. Between the supply voltage VDD and a ground, two diodes M3 andOELD are connected in series and the voltage at the node P2 becomes tohave a characteristic voltage Vc determined by the characteristic of thetransistor M3. Accordingly, the capacitor C1 stores a voltage differencebetween the node P1 and the node P2, which equals to a voltagedifference (V_(DATA) _(—) _(IN1)−Vc) between the initial data voltageV_(DATA) _(—) _(IN1) and the characteristic voltage Vc.

Since the transistor M3 in a diode connection between the gate and thedrain in this compensation step S2 operates as a diode, thecurrent-voltage characteristic curves of the transistor M3 are presentedas the curves G1 and G2 in FIG. 4A, and the current-voltagecharacteristic curve of the organic EL device OELD is presented as thecurve G0 in FIG. 4A. The driving conditions of the organic EL deviceOELD are determined at the cross point of the current-voltagecharacteristic curves of the transistor M3 and the organic EL deviceOELD. Accordingly, when the initial setting is performed in thecompensation step, the current deviation according to the characteristicdeviation of the transistor M3 is given by (I2-I1).

However, in a conventional case where the gate and the drain of thetransistor M3 are not connected, the typical current-voltagecharacteristic curves G3 and G4 shown in FIG. 4B show large deviationdepending on the voltage V_(GS) between the gate and the source. Thecurrent deviation according to the characteristic deviation of thetransistor M3 at the point where the driving conditions of the organicEL device OELD are determined becomes (I4-I3), which is greater than(I2-I1).

In a subsequent data voltage application step S3, the transistor M2 iscut off by setting the compensation signal SEL2 to have a low value andthe data voltage is applied to drive the transistor M3. At this time,since the characteristic voltage Vc is charged in the capacitor C1 inthe compensation step, the switching time of the transistor M3decreases. If the transistor M3 is driven, electric currentcorresponding to the data voltage flows into the organic EL devicethrough the transistor M3 such that the organic EL device emits light.

In the meantime, since characteristics of the organic EL devicesemitting red, green, and blue lights differ from each other, the area ofthe transistors M3 and the voltage values of the supply voltage VDD maybe independently determined for the respective R, G, and B pixels.

Even though the switching transistor M1 and the compensation transistorM2 are NMOS transistors and the driving transistor M3 is a PMOStransistor in the pixel circuit shown in FIG. 2 according to the firstembodiment of the present invention, the transistors M1, M2, M3 can bereplaced by other type transistors. Such embodiments will be describedwith reference to FIG. 5 to FIG. 10 hereinafter.

FIG. 5 is schematic circuit diagram of a pixel circuit according to asecond embodiment of the present invention and FIG. 6 is adriving-timing diagram of the pixel circuit according to the secondembodiment of the present invention.

As shown in FIG. 5, a pixel circuit according to the second embodimentof the present invention is substantially identical with the pixelcircuit according to the first embodiment except that the transistor M1for supplying electric current is a PMOS transistor. The driving-timingfor the pixel circuit according to the second embodiment issubstantially identical with the driving timing according to the firstembodiment except that the selection signal has a low value forselecting the scan line as shown in FIG. 6.

FIG. 7 is a schematic circuit diagram of the pixel circuit according toa third embodiment of the present invention and FIG. 8 is adriving-timing diagram of the pixel circuit according to the thirdembodiment of the present invention.

As shown in FIG. 7, a pixel circuit according to the third embodiment ofthe present invention is substantially identical with the pixel circuitof the first embodiment except that the compensation transistor M2 is aPMOS transistor. The driving timing of the pixel circuit according tothe second embodiment, as shown in FIG. 8, is substantially identicalwith the driving-timing of the first embodiment except that thecompensation signal has a low value for turning of the compensationtransistor M2.

FIG. 9 is a schematic circuit diagram of a pixel circuit according to afourth embodiment of the present invention and FIG. 10 is adriving-timing diagram of the pixel circuit according to the fourthembodiment of the present invention.

As shown in FIG. 9, a pixel circuit according to the fourth embodimentof the present invention is substantially identical with the pixelcircuit according to the first embodiment of the present inventionexcept that the current driving transistor M1 and the compensationtransistor M2 are PMOS transistors. The driving-timing of the pixelcircuit according to the fourth embodiment, as shown in FIG. 10, issubstantially identical with the driving-timing of the pixel circuitaccording to the first embodiment except that the selection signal has alow value for selecting the scan line and the compensation signal has alow value for driving the compensation transistor M2.

The pixel circuits and driving methods thereof according to the secondto the fourth embodiments will be apparent to the skilled in the artfrom the description of the first embodiment of the present inventionwith reference to FIGS. 2 to 4, and thus the description thereof will beomitted.

As described above, although the first to fourth embodiments of thepresent invention includes the three steps of the initialization, thecompensation, and the data voltage application, the initialization stepcan be ignored.

Even though the driving transistor M3 in the present invention isdescribed as a PMOS transistor, it may be an NMOS transistor. In case ofusing the NMOS transistor, the circuit and its driving will be apparentto the skilled in the art from the consideration of the first to fourthembodiments of the present invention, and the description thereof willbe omitted.

The present invention compensates the unevenness of the brightnesscaused by the characteristic deviation of the driving thin filmtransistors and decreases the switching time because the capacitor ischarged with a voltage in the compensation step.

While the present invention has been described in detail with referenceto the preferred embodiments, those skilled in the art will appreciatethat various modifications and substitutions can be made thereto withoutdeparting from the spirit and scope of the present invention as setforth in the appended claims.

1. An organic electroluminescent (EL) display comprising: a plurality ofdata lines for transmitting data voltages representing image signals; aplurality of scan lines for transmitting selection signals; a pluralityof compensation lines for transmitting compensation signals; and aplurality of pixel circuits provided at pixel areas defined by twoadjacent data lines and two adjacent scan lines, wherein each pixelcircuit includes: an organic electroluminescent device emitting lightcorresponding to amount of current applied thereto; a first switchingdevice for switching the data voltages applied to the data line inresponse to the selection signal applied to the scan line; a first thinfilm transistor connected to a supply voltage for supplying current tothe organic electroluminescent device according to the data voltageinputted to a gate thereof from the first switching device; a secondswitching device for switching in response to the compensation signalapplied to the compensation line such that the first thin filmtransistor performs a function of a diode; a first capacitor that isconnected between the gate of the first thin film transistor and a drainof the first switching device for maintaining the data voltage appliedto the gate of the first thin film transistor for a predeterminedperiod; and a second capacitor that is connected between the supplyvoltage and the drain of the first switching device for uniformlymaintaining the voltage applied to the gate of the first thin filmtransistor during application of the data voltage, wherein the firstcapacitor and second capacitor are connected in series between thesupply voltage and the gate of the first thin film transistor.
 2. Theorganic EL display of claim 1, wherein the compensation signal isapplied to the compensation line before the data voltage is applied tothe data line.
 3. The organic EL display of claim 2, wherein the datavoltage is applied to the data line after the compensation signalapplied to the compensation line is cut off.
 4. The organic EL displayof claim 1, wherein different levels of supply voltages are applied tothe first thin film transistors of red, green, and blue pixels.
 5. Theorganic EL display of claim 1, wherein the first switching device is asecond thin film transistor having three terminals, the three terminalsof the second thin film transistor including a gate connected to thescan line and other two terminals connected to the data line and thecapacitor, respectively, and the second switching device is a third thinfilm transistor including three terminals, the three terminals of thethird thin film transistor including a gate connected to thecompensation line and other two terminals connected to the gate and adrain of the first thin film transistor.
 6. The organic EL display ofclaim 5, wherein the first thin film transistor is a first conductiontype transistor and the second and the third thin film transistors aresecond conduction type transistors.
 7. The organic EL display of claim5, wherein the second and the third thin film transistors are differentconduction type transistors.
 8. The organic EL display of claim 5,wherein the first to the third thin film transistors are the sameconduction type transistors.
 9. The organic EL display of claim 1,wherein a terminal of the first capacitor is connected to the secondcapacitor and the drain of the first switching device.